3.11 Concentrations in mol/dm³ and titration calculations

Calculating concentrations in mol/dm3, converting between mol/dm3 and g/dm3, and using titration results to calculate an unknown concentration or volume.

Higher only Separate only AQA Edexcel OCR 1 min read

Concentration in mol/dm³

Key terms

Titration: a method for finding the exact volume of one solution that reacts with a known volume of another, using an indicator to show the end point.
Concordant results: titres within 0.10 cm³ of each other.

concentration (mol/dm^3) = moles / volume (dm^3)

So moles = concentration × volume (in dm³).

To convert between the two units of concentration:

mol/dm³ → g/dm³: multiply by M_\mathrm{r}
g/dm³ → mol/dm³: divide by M_\mathrm{r}

Example: a solution of sodium hydroxide (M_\mathrm{r} = 40) has a concentration of 20 g/dm³. In mol/dm³: 20 ÷ 40 = 0.50 mol/dm³.

Titration calculations

Work out the moles of the solution whose concentration you know (concentration × volume in dm³).
Use the balanced equation to find the moles of the other substance.
Divide by its volume in dm³ to find its concentration.

Flowchart for a titration calculation: moles of HCl from concentration and volume, the 1 : 1 ratio gives moles of NaOH, divide by its volume to get 0.0800 mol/dm3, then multiply by Mr to get 3.20 g/dm3

When the ratio is not 1 : 1

Example: 25.0 cm³ of sodium hydroxide is neutralised by 12.5 cm³ of 0.100 mol/dm³ sulfuric acid.

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

moles of H₂SO₄ = 0.100 × 0.0125 = 0.00125 mol
ratio 1 : 2, so moles of NaOH = 2 × 0.00125 = 0.00250 mol
concentration of NaOH = 0.00250 ÷ 0.0250 = 0.100 mol/dm³

Finding an unknown volume

Rearrange: volume (dm³) = moles ÷ concentration. Multiply by 1000 to get cm³.

Test yourself on this lesson

5 exam questions · 10 marks · with mark schemes and model answers

Start questions

Found a mistake?